1. Field of the Invention
The present invention relates to a nitride semiconductor, for example, which is used for manufacturing a semiconductor laser device or the like, a semiconductor device using the same, and manufacturing methods for the same.
2. Description of the Related Art
In recent years, III-V group compound semiconductors attract attention as a device material because of their various characteristics. Especially these materials are direct transition type ones and have a band gap width ranging from 1.9 eV to 6.2 eV, so only these materials provide light emitting in a wide region raging from a visible region to an ultraviolet region, and developments thereof as a material of semiconductor light emitting devices such as a semiconductor laser and light emitting diode (LED) are actively progressing. In addition to their wide band gap width, it can be expected that they have high electron saturation velocity and a high breakdown field, so that they are studied also in terms of applications to devices which operates in conditions where conventional devices with a Si— or GaAs— material cannot theoretically operate such as a high temperature operation, a high speed switching operation, and a high electric current operation.
Of these III-V group compound semiconductors, gallium nitride semiconductors such as GaN, AlGaN, and GaInN are materials which are advantageously applied to the devices and such a semiconductor device has been produced by laminating a gallium nitride semiconductor film on a surface of a crystal substrate or a crystal film. The crystal substrate (or the crystal film) has desirably bulk crystals of a gallium nitride compound, but manufacturing such a kind of bulk crystals is difficult, so the gallium nitride compound is formed by epitaxial growth on a substrate such as sapphire (α-Al2O3), silicon carbide (SiC), or the like in most cases.
However, there is a large difference in lattice mismatching and thermal expansion coefficient between the substrate material such as sapphire or the like and the gallium nitride compound, and lattice defects such as dislocation occur in a layer of the gallium nitride compound to relax distortion thereof. A lattice defect part serves as a center of non-radiative recombination, which emits no light even if an electron and a hole recombine, or as a leak part of electric currents, which causes damages of characteristics of the semiconductor devices.
Then, crystal growth methods have considered for removing the defects from the gallium nitride compound, and a growth technique, which utilizes a fact that little dislocation derived from seed crystals is in crystals growing in a transverse direction to the seed crystals used as a growth base, i.e., horizontally to a surface of a formed layer, is beginning to be applied to crystals of GaAs or GaN at present.
For example, Japanese Patent Laid-Open No. 10-312971 employs a method of forming a GaN layer on a sapphire substrate, forming a growth suppressing layer consisting of SiO2 (silicon dioxide) on a surface of the GaN layer, and growing crystals of GaN based on a GaN surface exposing through the growth suppressing layer. According to the method, the growth suppressing layer suppresses growth of the dislocation, and an amount of the dislocation which penetrates crystals and reaches an upper surface thereof (so-called threading dislocation) is decreased. However, there is dislocation which passes through an opening part of the growth suppressing layer and penetrates the crystals, and dislocation and defects are increased locally in a region which is above the opening part of the gallium nitride semiconductor layer.
Other methods include a method of forming many seed crystal parts on a GaN layer by means of pattern formation and growing crystals in a transverse direction based on the seed crystal parts to connect the crystals being grown in the transverse direction among the seed crystal parts, for example. However, also in the method, the dislocation may spread to an upper surface of the seed crystal parts, so the region which is directly over the seed crystal parts becomes a region locally having many dislocation and defects. Therefore, using these methods is insufficient for reducing the defects of the surface of the gallium nitride semiconductor on a substrate, which is a problem.
Furthermore, the transverse growth in these methods is an incomplete selective growth, and an upward growth also occurs as well as the transverse growth, so that a thickness is rapidly increased during fully performing the transverse growth, and this may result in bowing in a formed gallium nitride semiconductor layer. Then, the inventors of the present invention have tried to grow a gallium nitride semiconductor at a temperature higher than conventional methods so that the transverse growth should proceed dominantly, in order to obtain a thin layer thickness. As the growth temperature is higher, directivity of the growth direction becomes stronger and the transverse growth is further promoted, but now a defect called a hillock may occur in a layer surface. The hillock is a crater-like protrusion with a diameter of 70 μm-100 μm and a height of about 0.7 μm, and experiments have revealed that the hillocks tend to grow mainly right over the seed crystal parts (or the opening part of the growth suppressing layer). Defects may occur in the semiconductor layer being grown on the hillocks and this may damage characteristics of the produced semiconductor device. In the case of the semiconductor laser, when laser stripes are formed on the hillocks, there are problems of lowering reliability such as laser static characteristics and a life of the laser.